1. Field of the Invention
This invention is in the field of methods for the production of optical fiber, particularly in the field of methods for monitoring the drawing and coating of such fiber.
2. Description of the Prior Art
Optical fiber is currently generally produced by drawing from a reservoir of fluid glass, such as the hot end of a rod, often called a preform, or from a crucible. Process control in practice has been generally limited. Most commonly, only the fiber diameter is monitored, and kept constant by adjusting drawing speed and, in specialized cases, the temperature of the hot zone. A method does exist, however, for detecting the presence and location of physical defects in optical fibers simultaneously with the drawing of the fiber. In this method, disclosed in U.S. Pat. No. 4,021,217, the scattering of light directed at the fiber approximately perpendicularly to the fiber axis is detected, the scattered light signaling a defect. The prior art also contains a method for detecting localized defects in bundles of optical fibers, by directing light axially into the bundle and observing the light leakage from the bundle (R. H. Mancini, IBM Technical Disclosure Bulletin, Vol. 14, No. 11, 3294 (1972)). This method is said to be adaptable to the inspection of single fibers during drawing, and thus would provide the same information as the technique disclosed in U.S. Pat. No. 4,021,217. Both these techniques rely on the observation of the spatial location of the source of scattered light for determining the presence and location of localized defects in the fiber.
Such a crucial fiber characteristic as transmission loss per unit length, however, can only be determined after drawing has been completed, since such a measurement requires access to one or both ends of the fiber. One of the methods that is used for making such a loss measurement, and which, in addition, also yields information as to the presence and location of physical defects in the fiber, is optical time domain reflectometry (OTDR). In OTDR short pulses of light are directed onto one end of the fiber user test, and consequently these pulses travel through the fiber away from the launching end. A small part of the light in the pulses is scattered from the ever-present statistical fluctuations of the refractive index of the fiber material, so-called Rayleigh scattering. Some light also is scattered by localized physical defects as well as by the far end of the fiber. Part of this scattered light is guided back through the fiber and exists through the launching end. This light is then detected, generally by means that convert the optical signal to a corresponding electrical one, which, after processing, can, for instance, be displayed on the screen of an oscilloscope as a trace of intensity of backscattered light vs. time elapsed since launching of the pulse. Because the speed of light in the fiber is known, this trace is equivalent to one of intensity vs. distance travelled in the fiber. Since the amount of light scattered at any point in the fiber is strictly proportional to the light intensity at that point and the scattering power of the fiber at that point, the backscattered light can be used to measure the optical power at each point in the fiber, and thus the rate of decay of the optical power in the pulse can be measured. This, however, is just the transmission loss of the fiber. A discussion of the theory underlying this measurement technique can be found for instance in M. K. Barnoski et al., "Optical Time Domain Reflectometer," Applied Optics, Vol. 16, 2375 (1977).
In summary then, the relevant prior art contains a method for measuring the local transmission loss, including the presence and location of structural defects, in optical fibers which have at least one end available such that light can be coupled there into and out of the fiber. This means that the method can be used only after completion of the drawing process. The prior art also contains methods for detecting the presence and location of structural defects in the fiber as the fiber is being drawn. But no method exists that allows the determination of both transmission loss and the presence and location of defects during the process of drawing the fiber.